4 Answers
4

As it stands, that 'border' between FL600 and above is undefined, and unregulated.

There is no international agreement on the vertical limit of
state sovereignty. Today, the need to settle this gap in the law is
growing in importance.

More and more states are developing their own domestic
space launch capability. Few of these new space powers will be
able to freely access space, or utilize the most efficient launch
azimuths, if neighboring states can claim sovereignty up to even
62 miles (100 kilometers). They will have even more difficulty
returning objects to Earth if the boundary is set at that altitude.
Even the United States and Russia are facing limitations on their
ability to freely access space. Setting a low vertical limit on state
sovereignty will ensure all states have equal access to space.

Interestingly, there is no international law (or agreement) defining the vertical boundary of a nation’s sovereign territory/airspace (the boundary between outer space which is not subject to national jurisdiction and national airspace). Obviously, there is a “white” unregulated area that should be defined, not at least from national security concerns. Presently the airspace between FL 600 and up to the border between the earth’s atmosphere and outer space (generally an altitude of approximately 100 km (62 mi) (the Kármán line)) is not regulated.

It should be also said that the actual limit is much lower and starts with 30 km.

Even the current best Surface-to-Air Missiles have a height limit of approximately 25 km. Normal combat planes cannot climb for a horizontal speed higher than 20 km. While planes can take a "running jump" by going full throttle upward (e.g. the F-15 up to 30km), they cannot target well and it gets worse with every km going up.

Some powers have the MiG-25, but those planes will be of limited value against targets over 25 km (this includes the SR-71 which is a spy plane and without weaponry). Superpowers may have advanced satellite weapons available, but they need too much preparation, are extremely pricy and are too valuable to send against a target because it tells the others much too much about the capabilities of a nation.

So if you climb up with a stratopheric balloon over 30 km, every nation on earth is practically helpless. I don't need to say that such a stunt would really tick off a nation and is by no means recommendable.

$\begingroup$Current antiballistic missile systems have no trouble reaching altitudes higher than 30 km. Proliferation of those is limited, but increasing.$\endgroup$
– HobbesSep 4 '17 at 9:37

1

$\begingroup$@Hobbes It may be counterintuitive, but ABMs are unusable for the task of intercepting a weather balloon over a nation. The task of an ABM is to intercept very fast moving targets which are still far, far away during the start phase (thousands of kms). This means during the start ABMs are giving everything to increase velocity as fast as possible and they are moving out of the atmosphere because this is the only way to gain enough speed. So they are rising extremely fast and during this phase their control system cannot adjust their course enough to hit a target only hundreds of kms away.$\endgroup$
– Thorsten S.Sep 4 '17 at 17:11

$\begingroup$There's clearly some understanding there's a vertical limit, even if it's not concretely defined, or the US and Soviets/Russians would've been yelling at each other over airspace violations since the 1960s.$\endgroup$
– ceejayozSep 3 '17 at 22:07

$\begingroup$@ceejayoz: Space lacks¹ air and is therefore not airspace. US satellites did/do not pass through Soviet/Russian airspace and vice versa. (¹ to any significant extent - less than required to support an aircraft)$\endgroup$
– RedGrittyBrickSep 3 '17 at 22:13

$\begingroup$If there's not a defined international standard for the minimum molecules of air per square meter, that distinction is really pretty meaningless.$\endgroup$
– ceejayozSep 3 '17 at 23:18

$\begingroup$@ceejayoz, the point is, travelling vertically from a nation's territory you pass from national airspace into space without passing through anything internationally recognised as international airspace. The FAI, IOP and others use the Kármán line with reference to the limit beyond which orbital forces predominate over aerodynamic forces.$\endgroup$
– RedGrittyBrickSep 3 '17 at 23:53

No, there is no defined height limit to national airspace, but there is an upper speed limit to national airspace, or should we say, an interlock between speed and altitude.

Any aircraft that must generate net downlift in order to not gain altitude can be safely to be orbiting rather than flying.

And for all practical purposes, all orbital flight is considered to take place in international airspace (or in cases where the altitude is so high that the atmosphere is truly neglible, you may prefer to call it international spacespace.)

The higher the altitude and thus the further from the earth's gravitational center, the lower the speed required for a body to be considered to be orbiting rather than flying, and thus safely beyond the limits of any national airspace.